In order to cause a piece of subscriber equipment (e.g., telephone) to ring, thereby indicating the presence of an incoming call, an AC (ringing) voltage, which rides on a prescribed DC bias (+/-48 VDC) is customarily applied to the customer's line circuit from a ringing generator installed in the telephone central office. Termination of the ringing voltage, in response to detecting that the customer has answered the call, is termed ring-trip. The magnitude of the ringing voltage may vary over a substantial range (e.g., 70-140 VRMS) and the frequency of the ringing voltage may lie in a range of from 16-66 Hz. The loop current through the line may vary from twenty milliamps for a long line circuit up to one hundred milliamps for a short line circuit.
Due to these substantial parameter and polarity variations, currently commercially employed subscriber line circuit interface chips typically employ high precision components, whose values must be tailored in accordance with the length of the line circuit and the magnitude and frequency of the ringing voltage. Moreover, since such circuits employ a ground reference, they require additional components to protect the chip from line fault voltages. Other forms of ring-trip circuits use expensive, custom wound relay coils, which require associated resistor and capacitor components that are selected according to the length of the line circuit and the frequency of the ringing voltage.